Compound semiconductor modified surface by use of pulsed electron beam and ion implantation through a deposited metal layer

ABSTRACT

Thermally sensitive at elevated, near melting point temperature, compound semiconductor materials single crystals including Group III-Nitride, other Group III-V, Group II-VI and Group IV-IV are produced by a variety of methods. When produced as single crystal layers by epitaxy methods or is necessary to expose them to elevated temperatures or ion implanted to the non crystalline state, or their electrical or optical properties are modified, large numbers of crystal defects on the atomic or macro scale may be produced, which limit the yield and performance of opto- and electronic devices constructed out of and grown on top of these layers. It is necessary to be able to improve the crystal quality of such materials after being exposed to elevated temperature or ion implanted or modified by the presence of impurities. It is necessary, particularly for opto- and electronic devices that only the surface of such materials is processed, improved and thus the modified surface product. Generally, as shown in FIG.  1,  the thermally sensitive compound semiconductor layer is first coated with a metal layer of approximate thickness of 0.1 microns. Next, the volatile component of the compound semiconductor is ion implanted through the metal layer so as to occupy mostly the top 0.1 to 0.5 microns of the compound semiconductor layer. Co-implantation may be used as well to improve the surface. Finally, through a pulsed directed energy beam of electrons with a fluence of approximately 1 Joule /cm 2 , the top approximately 0.5 microns acquire a level of the deposited metal and are converted into a single crystal with improved properties such as reduced defect density and or electrical dopant (FIG.  1 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Divisional Application related to patent application Ser. No.11/014,304 filed 2004, Dec. 16 and to Provisional Patent Application No.60/531,001 filed on Dec. 19, 2003

BACKGROUND ART

This application relates to the product of the use of a pulsed electronbeam by itself or in combination with other steps to performsemiconductor processes particularly on compound semiconductors of theGroups III-V and II-VI, as well as IV-IV, of the Periodic Chart of theelements. These crystalline materials are normally synthesized at hightemperatures and even very high pressures (Reference #1). Typically atthe melting point and standard pressure, the partial pressure of theGroup V (or VI) element is high, such that special precautions arerequired to keep the crystalline imperfections low. For example, in thecase of GaN, at the extrapolated melting point of 2518 C., the partialpressure of Nitrogen over the liquid GaN is approximately ten thousandatmospheres (Reference #5), an exceedingly difficult condition toachieve on a practical scale. This is also important in the case ofepitaxial layer growth of compound semiconductors, where typicallytemperatures of 600-1000 C. are used to form many technologicallyimportant alloys and devices (Reference #2).

Given that these compound semiconductors are sensitive and prone todecomposition at higher than ambient temperatures, any technique whichmust improve the crystal quality needs to be very fast and below thetime it takes to break apart a molecular bond, or typically below amicrosecond. Light energy transfer techniques such as from a flash lampare slow compared to a pulsed electron beam and produce undesirabletemperature rise in the entire substrate.

Pulsed electron beams have been used in the past to anneal ionimplantation damage in Silicon wafers as large as 100 mm OD. The pulsedelectron beam melts the Silicon wafer surface at 1410 C. and thecrystallinity of the top micron or so is repaired. The pulsed electronbeam is typically of 0.1 microseconds in duration, produced by acapacitor discharge where the electron beam is accelerated through ahigh voltage field and directed to the substrate. Alternative pulsegeneration systems may also be used. The beam total is in the range ofthousands of amperes and the electrons acquire 10 to 200 KeV energywhile a high degree of control is possible.

At the other extreme such as when the pulsed electron beam is highlyfocused, it can lead to very rapid and localized temperature rise andthus to vaporization and consequently deposition of the target materialon a substrate (Reference #6). However this is the opposite process andnot as likely to lead to a single crystalline material, particularly forcompound semiconductors which do not have a defined melting point andmay decompose on heating.

Ion implantation into a compound semiconductor crystal material is wellknown that at levels in the 10⁺¹⁶/cm² range and higher will result in anamorphous phase (Reference 4). In the case of GaN, this amorphous phasewill recrystallize into a polycrystalline material at annealingtemperatures below about 1100 C. It is necessary though to anneal outthe implantation damage at temperatures exceeding two thirds theextrapolated melting point of GaN which is 2518 C. and at ten thousandatmospheres of Nitrogen pressure (Reference #5). So, not only is a hightemperature required but also a high volatile component (in the case ofGaN it is Nitrogen) overpressure corresponding to the phase diagram.Additionally, through ion implantation it is possible to reach veryhigh, non equilibrium concentrations to allow certain alloys to formthat are not possible for example in furnace. For example the atomicpercent of Nitrogen into GaAs or Arsenic into GaN are limited. Throughion implantation, these compositional limits may be extended.

It is a basic part of the method used and the achieved resultingsurfaces that a metal is first deposited. That is, in the case ofthermally sensitive at melting point temperature materials, a protectivelayer, and for these materials and the pulsed electron beam, mostappropriately, a metal is used. In this unique combination, the metalcaps the material during the high temperature produced by exposure tothe pulsed electron beam, and does not allow it to decompose.

Blue LED's and lasers are of particular importance to not only tocomplete the optical spectrum but for very high density Digital VideoDisk and other optical storage applications. A particularly difficultproblem for these materials relates to the substrate necessary to growthin layers that comprise the laser, L.E.D or other electronic oroptoelectronic device (Reference 3). The substrate performs severalfunctions from providing the mechanical support, to thermal management,to allowing epitaxy to take place through its crystal structure anddimensions, to being either electrically active through impurity dopingor insulating again possibly through impurity doping. Group III-Nitridesubstrates are the ideal materials for homoepitaxy of these materials.It is known that the growth of large (over a few mm in diameter) singlecrystal substrates, is extraordinarily difficult to achieve compared toGaAs or InP, for example, which are commercially available to 150 mmOutside Diameter (OD).

SUMMARY OF THE INVENTION

The product of a method of modifying the crystal quality of a compoundsemiconductor material which is comprised of the following:

A. Provide a layer of a compound semiconductor material, whereincompound semiconductor material comprises a metal component and anon-metal volatile component and layer comprises a top surface;

B. Place said layer of said compound semiconductor material into a metaldeposition tool and deposit a layer of metal upon said top surface;

C. Place said layer of said compound semiconductor with said depositedlayer of said metal into an ion implantation tool and implant non metalcomponent into said layers;

D. Place said layer of said compound semiconductor with said depositedmetal layer and said implanted non-metal component into a pulsedelectron beam tool and expose said layers to a pulsed electron beam.

Additionally:

The product of method above A-D, wherein said compound semiconductorlayer is deposited on a foreign substrate.

The product of method above A-D, wherein said compound semiconductorlayer comprises Group III-Nitrides, other Group III-V, Group II-VI orGroup IV-IV.

The product of method above A-D, wherein said compound semiconductorlayer is deposited by an epitaxial deposition method selected from agroup consisting of Hydride Vapor Phase Epitaxy, (HVPE), Metal OrganicVapor Phase Epitaxy (MOVPE), Molecular Beam Epitaxy (MBE) or similartechnologies.

The product of method above A-D, wherein said metal component layercomprises Aluminum, other Group III, or Group I, or Group II, ortransition or lanthanide or any other metal element of the PeriodicChart. Additionally, more than one metal element may be used together asone layer over another.

The product of method above A-D wherein said implanted non-metalcomponent comprises of Nitrogen ions, or other volatile element.

The product of the method of above A-D, wherein the energy of saidimplanted non-metal component is selected to be sufficient so as to gothrough said deposited metal component layer.

The product of method above A-D, wherein the amount of said implantednon-metal component is selected to be sufficient to provide an excess ofsaid non-metal component into said compound semiconductor layer.

The pulsed electron beam comprises an energy of a range approximately0.01 Joule per cm² to 2 Joules/cm², of a pulse duration of less than amicrosecond and a diameter of at least 3 mm. The pulsed electron beammay be used sequentially more than once and may be moved over thesurface of said layers in a controlled manner. Finally, said pulsedelectron beam is stationary and said layers are exposed to said electronbeam by moving said layers in a controlled manner.

The product of method above A-D, wherein said exposing of said layers tosaid electron beam occurs under a background gas pressure and whereinsaid gas comprises Nitrogen, or Oxygen or other gas.

LIST OF FIGURES

FIG. 1. Modified Compound Semiconductor Surface block diagram.

DESCRIPTION OF THE INVENTION EXAMPLES

The pulsed electron beam process requires a conductive surface in orderto be highly uniform. This is done by evaporating a metal such asAluminum or other Group III metal or other metal such as a noble metalin the range of a ten to a thousand, or more, nanometers (FIG. 1). Thisprovides the basis to improve the crystallinity of a Group III-Nitride,other Group III-V, Group II-VI or Group IV-IV layer or substrate surfacebut also to alter the electrical properties of the substrate or layer asfollows:

Example 1 Pulsed Electron Beam Through Deposited Aluminum Layer

A. Deposit a high purity layer of Aluminum metal in the range of 0.01 toseveral micrometers thick. This can be by ebeam evaporation or by aChemical Vapor Deposition or other technique, as long as high purity isachieved. The use of Aluminum is significant not only because it is aGroup III metal and highly conductive but also because Al_(x)Ga_(1-x)Nlayer alloy can be produced. At concentrations below about 1 atomicpercent, the Aluminum will not alter significantly the materialproperties of GaN.

B. Use a pulsed electron beam as wide as 100 mmOD, generated bycapacitor discharge. Typically, an electric field of approximately 10 to100 KiloVolt is used, a total of 1-50 Kilo Amperes, with a pulse widthunder a microsecond, typically of 80 to 500 nanoseconds, resulting in anenergy fluence from 0.1 to 10 Joules per cm². The electron beam pulsemay be repeated as necessary to optimize the results.

C. The result of the pulsed electron beam is to raise the surfacetemperature from ambient to approximately 2300 Celcius, the boilingpoint of Aluminum metal. As the surface temperature rises by the energytransfer, a portion of the Aluminum may evaporate while another portionof the Aluminum atoms move from the surface into the underlyingmaterial. In the case of Aluminum, given its reactivity, it can alloywith the Group III-Nitride substrate material. Since the pulse issufficiently short, decomposition does not occur and is furthercontrolled by the capping Aluminum layer.

D. The resulting Al_(x)Ga_(1-x)N alloy as well as the surface to somedepth, is now of lower EPD as a result of filling the voids and otherdefects and defect annihilation by solid state diffusion and alloying.By use of higher energy, such as going from under 20 KV to up to 1 MV ormultiple pulses, the depth of Aluminum diffusion, defect annihilationand alloying can be controlled as needed for electrical oroptoelectronic device applications

E. Any remaining Aluminum layer or even the alloy generated at the topsurface, can be etched off leaving an improved crystalline surface andmore suitable for example for high yield laser layer deposition.

Example 2 Pulsed Electron Beam Through Deposited Indium Layer

A variation of Example 1 is where Indium metal is used instead ofAluminum. This is particularly important since in this case, theresulting In_(x)Ga_(1-x)N layer, which has a larger crystal latticefurther improves the GaN crystal structure by expansion.

Example 3 Pulsed Electron Beam Through Deposited Boron Layer on GroupIII-Nitride

Another variation of Example 1 is to use Boron which has the advantageto produce a better metal contact due to the higher energy bandgap thanGallium Nitride itself.

Example 4 Pulsed Electron Beam Through Deposited Dopant Layer

An important variation of this process is to deposit a dopant metalelement for a Group III-Nitride such as Group II metals Magnesium, Zinc,Cadmium or Beryllium and Group VI metallic Tellurium. A two metal systemmay be used with a Group III deposited on top of and either Group II orGroup VI metal. Upon pulsed electron beam exposure the dopant element isdriven into the substrate at some depth and as necessary, throughmultiple pulses to achieve better distribution uniformity. Additionally,given the very surface temperature achieved by the pulsed electron beamthe dopant concentration may reach a higher level as well as higherelectrical activation and greater depth than by any other thermaltechnique.

Example 5 Pulsed Electron Beam Through Aluminum (or Indium) afterNitrogen Ion Implantation

In an additional effort to repair the GaN crystal structure after step 1above and prior to use of the pulsed electron beam, a dose of Nitrogenis ion implanted through the Aluminum (or Indium) layer. The dose ischosen to be sufficient to provide additional Nitrogen to bond withvacant Gallium to Nitrogen bonds as well as the additional depositedAluminum or Indium and thus again fill the voids by expanding thecrystal lattice.

Example 6 Nitrogen Containing Alloy Formation

Ga_(x)In_(1-x)N_(y)As_(1-y) alloys are technologically important forlaser used optical fiber systems but are extremely difficult to producewith high Nitrogen content due to the thermodynamic instability atreversible growth conditions. The pulsed electron beam process canaffect such high Nitrogen content alloy formation since it is not anequilibrium growth process.

In order to produce Ga_(x)In_(1-x)N_(y)As_(1-y) alloys, start with a GaNsubstrate and deposit a layer of Arsenic and a layer of Indium and thenfollow the above steps in Process I with a pulsed electron beam process.This forms a top layer with high Nitrogen content which can then be usedfor epitaxy of these alloys.

Example 7 Pulsed Electron Beam Processing of Group II-VI Materials

As in the above Process I to III a pulsed electron beam is used toimprove the crystallinity of various Group II-VI materials such as ZincSelenide, Zinc Oxide and others as well as to effect doping p or n typeby depositing the proper metal. For example, a Group I or Group V metalmay be used to effect p-type doping.

Example 8 Pulsed Electron Beam of Group IV-IV Materials

Group IV-IV materials are becoming technologically very important foreven more stringent applications than Group III-V semiconductors.Silicon Carbide has greater radiation tolerance and higher operatingtemperature and voltage range than the Group III-V semiconductors.Typically SiC single crystals formed by expensive sublimation processeshave undesirable micro defects, known as micro pipes, which reduce theyield of processed devices. By using a pulsed electron beam process asdescribed above these defects may be reduced. Additionally, bydepositing a dopant element such as Boron or Aluminum or a Group V andexposing to a pulsed electron beam the Group IV-IV can be doped moreeasily than by thermal processing as in a furnace. Finally, ionimplantation may be used to further improve dopant level by usingSilicon ion for example.

Example 9 Pulsed Electron Beam of Metal Contacts

Metal contacts to compound semiconductor materials need to be ohmic, aslow as possible electrical resistance and adherent. Additionally thesurface only where the contacts are must be heated not the entire devicestructure with the substrate, which is not possible in a furnace. Theseproblems are overcome by use of the process. This is achieved by firstdepositing the metal(s) and then exposing to the pulsed electron beam atthe correct fluence to reach high enough surface temperature to reduceseries resistance and improve adhesion.

PREFERRED EMBODIMENTS

-   1. This application, in part, relates to currently available growth    process which produce free-standing GaN, other Group III-V, Group    II-VI and Group IV-IV substrates. These crystals are temperature    sensitive particularly at or above their melting point. When grown    by lack of availability or other requirement on a lattice mismatched    material, the crystal defects are very large in number, i.e.    10⁺⁸-10⁺¹⁰/cm². In the case when epitaxy is required to grow Group    III-Nitride devices such as lasers, the yields are very poor. The    product of method in application Ser. No. 11/014,304 is the    reduction by at least two orders of magnitude of surface defects    that is necessary to produce commercially useful substrates of this    type.-   2. The use of deposited metal as an encapsulating and material    modifying layer is important in several ways. First, it is necessary    to spread out the electron beam. Second it is sacrificial and can be    sputtered off during volatile element implantation or blown off by    pulsed electron beam. Third it can be reacted out to the metal    containing alloy. Fourth, during the volatile element implantation,    the entire substrate may be heated to about 500 C., which reduces    the radiation damage. Fifth, the use of the metal layer effectively    slows down the implanted Nitrogen ions and thus the radiation damage    is minimized. And, Sixth, in the case of a dopant element such as a    Group II for Group III-V or a transition or rare earth, the    electrical as well as optical crystals may be altered.-   3. The use of ion implantation is standard in semiconductor    technology. However, it is also well known that radiation damage due    to high energy of the implantation process requires a high enough    temperature to anneal out, which is estimated at ⅔ of the melting    point (Reference 5). In the case of Gallium Nitride that is around    1650 Celcius. Additionally, N⁺ implantation may result in a porous,    amorphous material with gaseous inclusions due to decomposition and    therefore not obvious. Even higher levels of N⁺ implantation may    result in a higher concentration of interstitial Nitrogen, which    would produce a higher Nitrogen overpressure which is necessary to    anneal out the defects to a greater degree but then at an even    higher temperature. By exposing only the top surface to the pulsed    electron beam, the rest of the material is not heated which is an    advantage in processing devices.-   4. Given that these compound semiconductors are sensitive and prone    to decomposition at higher than ambient temperatures, any technique    which can improve the crystal defects of grown wafers as compared to    a boule, needs to be very fast, at sub microsecond duration, such as    the pulsed electron or laser techniques.-   5. In the case of ion implantation damage, a directed energy beam    such as a pulsed electron beam has been demonstrated to anneal out    the damage, at the appropriate energy level or fluence, as energy    per cm². The voltage used as well as the energy fluence, in Joules    per cm², affect the charachteristics of the beam such as surface    penetration. Additionally, the beam must be controlled to be as    uniform as possible to achieve uniformity of heating and thus    crystallinity repair. The fluence required for this application is    in the order of 1 Joule per cm².-   6. When the pulsed electron beam is highly focused, such as    approximately 1 mm², the highly focused beam can lead to very rapid    and localized temperature rise and thus to vaporization and    consequently deposition of the target material on a substrate    (Reference 6). In this application, a wide beam of the necessary    fluence, is required to produce annealing and reaction to relieve    the radiation damage on a larger scale and be of commercial value.    The result of the directed energy beam such as the pulsed electron    beam is to raise the surface temperature from ambient to well over    1000 C. depending on the fluence and other factors such as energy    coupling to the surface. As the surface temperature rises by the    energy transfer, the Aluminum or other metal atoms from the    deposited and melted Aluminum or other metal layer on the surface,    diffuse rapidly into the underlying material. In this case,    Aluminum, given its reactivity, it can alloy with the Gallium    Nitride substrate material and in the process compress the    interstitial Nitrogen to very high pressure and temperature. Since    the pulse is sufficiently short, decomposition does not occur and is    further controlled by the capping Aluminum Nitride and Aluminum    layer. This AlGaN alloy formed from the surface to some depth, is    now of lower crystal defect density as a result of filling the voids    and other defects and defect annihilation by solid state diffusion,    alloying and very high temperature and pressure. This is effectively    then is also a surface polishing technique.-   7. By use of multiple directed energy pulses, in the case of AlGaN    by HVPE, the temperature rise as well as depth of Aluminum    diffusion, increase. This leads to greater defect annihilation and    the crystallite size increases by several fold and the X Ray    Diffraction half width decreases compared to the single pulsed    layers.-   8. By use of a dopant metal layer alone or in combination with    Aluminum for example in a Group III-Nitride material, the electrical    and or optical properties of the material may be modified. Again    this is achieved only at the surface without exposing the entire    device structure to the high temperature. Furthermore, the impurity    level may be controlled and reach a higher level than that achieved    from a furnace, due to the higher temperature achieved by the pulsed    electron beam.-   9. In the case of metal contact treatment by the pulsed electron    beam the top surface only is exposed avoiding the possible    decomposition problems of furnace treatment.-   10. In the case of alloy formation where by ion implantation of the    volatile non metal component a higher concentration and improved    crystal quality may be achieved than by another growth technique.

While the invention has been described in terms of certain preferredembodiments and material systems, modifications obvious to those withordinary skill in the art may be made without departing from the scopeof the invention.

REFERENCES (Already Provided in application Ser. No. 11/014,304)

-   1. Gallium Arsenide Technology, D. K. Ferry, Editor, Howard W. Sams    & Co. publishers, 1985, p. 47-105.-   2. Organometallic Vapor-Phase Epitaxy, Theory and Practice, by    Gerald Stringfellow, Academic Press, Inc. publishers, 1989, p. 1-14.-   3. Geppert, L., “The Great Gallium Nitride Gamble”, IEEE Spectrum,    January 2004, pp. 52-59.-   4. Tan, H. H., et. al., “Annealing of ion implanted gallium    nitride”, Applied Physics Letters, V.72, Number 10, p. 1190-2-   5. Williams, J. S., Rep. Prog. Phys. 49, p. 491, (1986).-   6. Pulsed Electron Deposition System by NEOCERA, Beltsville, Md.,    20705

1. A top surface modified compound semiconductor material product formed by a method comprising: providing a layer of said compound semiconductor material, wherein said compound semiconductor material comprises a metal component and a non-metal component and said layer comprises a top surface; placing said layer of said compound semiconductor material into a metal deposition tool and depositing a layer of metal component upon said top surface; placing said layer of said compound semiconductor with said deposited layer of said metal component into an ion implantation tool and implanting non metal component into and through said metal layer; placing said layer of said compound semiconductor with said deposited metal layer and said implanted non-metal component into a directed pulsed electron beam tool and exposing said layers to said energy beam.
 2. The product of claim 1 wherein the said semiconductor material is a Group III-V material.
 3. The product of claim 1 wherein the said semiconductor material is a Group II-VI material.
 4. The product of claim 1 wherein the said semiconductor material is a Group IV-IV material.
 5. The product of claim 1 wherein the said metal layer is a Group III element.
 6. The product of claim 1 wherein the said metal layer is a Group I element.
 7. The product of claim 1 wherein the said metal layer is a Group II element.
 8. The product of claim 1 wherein the said metal layer is a transition metal.
 9. The product of claim 1 wherein the said metal layer is a lanthanide metal.
 10. The product of claim 1 wherein the said metal layer is any other metal in the periodic chart of elements.
 11. The product of claim 1 wherein the said metal layer is a combination of two or more metals.
 12. The product of claim 1 wherein the said metal layer is a combination of Group II and Group III on a Group III-V material.
 13. The product of claim 1 wherein the said metal layer is a combination of Group I and Group II on a Group II-VI material.
 14. The product of claim 1 wherein the implanted element is a non metal.
 15. The product of claim 1 wherein the said implanted element is Nitrogen.
 16. The product of claim 1 wherein the said implanted element is Carbon.
 17. The product of claim 1 wherein the said implanted element is Silicon.
 18. The product of claim 1 wherein the said implanted element is Oxygen.
 19. The product of claim 1 wherein the said implanted element is any other nonmetal element of the periodic chart of elements. 